Study On Laser Ultrasonic Detection Of Nitriding Layer Thickness Of Metal Materials

Nitriding is one of the surface heat treatment processes widely used in manufacturing industry.After nitriding treatment,the surface wear resistance and the contact fatigue strength of materials of 40 Cr steel can be improved effectively.The thickness of nitriding layer is an important basis and parameter to evaluate the results of nitriding process.Traditional methods,such as hardness method and metallographic method,are often used to detect the thickness of nitriding layer,but these methods are destructive and unrepeatable.Therefore,it is very important to realize a rapid and accurate nondestructive testing method of nitriding thickness of 40 Cr steel.In order to realize the nondestructive testing for nitriding thickness of 40 Cr steel,a laser ultrasonic surface wave nondestructive testing method was proposed.The surface wave is excited by a pulse laser on the metal surface,and the signals are received by the laser interferometer.The whole testing process has the non-contact,non-destructive and accurate characteristics,thus the laser ultrasonic method with non-destructive characteristics for the thickness of nitriding layer owns a great application prospect.The specific research contents of this paper include the following aspects:Chapter one: Introduction.Firstly,the research background was introduced,and the development and application of laser ultrasound were briefly summarized.Secondly,the research status of the laser ultrasonic surface waves was summarized.Finally,the main research contents of this paper were proposed.Chapter two: Ultrasonic theory in elastic solid.Firstly,the mechanism of laserinduced ultrasound was discussed.Secondly,the wave equation and particle trajectory equation of surface wave in elastic medium were deduced,and then we described the propagation characteristics of surface wave.Finally,the basic theory of laser-induced ultrasound was introduced in detail.Chapter three: Finite element method for Laser ultrasonic.In this chapter,the basic steps of the finite element simulation of laser-induced ultrasound were introduced firstly,and the important parameters of model were selected.Secondly the temperature field distribution and the whole ultrasonic wave pattern of the laser action area in isotropic materials were analyzed.Finally,the layer / matrix model with gradient elastic modulus was solved by the finite element method,and the effect of the thickness and the difference gradient elastic modulus on disperse were discussed.The results show that the surface waves are affected by the depth and the elastic modulus,which provides a theoretical basis for surface wave detection of nitriding layer.Chapter four: The destructive evaluation of the thickness of nitriding layer on metal materials.Firstly,the quenching and tempering process and the ion nitriding process of nitriding samples were introduced.Secondly,the thickness of nitriding layer was measured by hardness method and metallographic method,which provides data support for laser ultrasonic detection of the thickness of nitriding layer.Finally,the phase composition of nitriding layer surface was analyzed.Chapter five: Nondestructive Testing of the thickness of nitriding layers on metal materials.Firstly,the work and testing principle of laser ultrasonic testing system were introduced.Secondly,the laser ultrasonic system was used to test 40 Cr steel samples with different nitriding time,and the surface wave signals on nitriding samples was obtained.Then,the dispersion curve of samples with different nitriding time were obtained by two-dimensional Fourier transform.Finally,the dispersion curves of different nitriding samples were compared.The results show that the thickness of the nitriding layer is limited by the thickness of the nitriding layer.When the thickness of the nitriding layer is less than a certain value,there is a certain corresponding relationship between the propagation speed of the surface wave and the thickness of the nitriding layer.Through this corresponding relationship,the thickness of nitriding layer can be approximately obtained by this correspondence.Chapter six: Summarizing the research results of the thesis,and expounding the shortcomings and improvement methods of the thesis.